The present invention relates to oil-free/oil-less air compressors. It finds particular application in conjunction with oil-free/oil-less air compressors with polymeric piston rings having a predetermined, non-circular shape in the free state and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.
Oil lubricated air compressors use metallic piston rings to seal the gap between a piston and a round cylinder bore. Cylinder bores, however, are not perfectly round due to machining, assembly, and operational factors. The ability of a piston ring to conform to cylinder bore distortions impacts the ring's ability to seal. When installed in a cylinder bore, piston rings are typically compressed (“squeezed”) radially. Due to the ring's elastic nature, it wants to revert to its free state shape; thus, the ring exerts pressure on the cylinder bore. This “elasticity” or “internal tension” of the metallic ring helps it conform to the bore.
The free state shape of the ring affects the ring's “internal tension.” A metallic piston ring is typically made with a noncircular free state shape. A noncircular free state shape improves the ring's ability to conform to the cylinder bore. Metallic rings, however, cannot be used with oil-free/oil-less air compressors. Oil-free/oil-less air compressors do not provide lubrication required by metallic rings during operation. As a result, conventional oil-free/oil-less air compressors employ self-lubricating piston rings.
Piston rings for oil-free/oil-less air compressors are typically manufactured from round sintered tubes made of relatively soft polymeric materials such as polytetrafluoroethylene (PTFE) based materials. Due to the nature of the material, however, soft polymeric rings lack sufficient internal tension to conform the ring to cylinder bore distortions. Soft polymeric rings, therefore, must rely on the gas pressure developed during compressor operation and the flexibility of the soft polymeric material to try to conform the ring to the shape of the bore.